1. Field of the Invention
The present invention concerns an improved process for performing HC1-membrane electrolysis.
2. Background Information
According to the state of the art HCl is electrolysed by feeding an aqueous solution of hydrochloric acid into a plurality of individual cells each of which is separated by a diaphragm into two compartments. 30-45 of the individual units are put together to a stack, which is called electrolyser. Each cell of said electrolyser is connected to the catholyte and to the anolyte circulation system in a way, that the anolyte acid flows through all anode compartments in parallel and the catholyte acid flows through all cathode compartments in parallel. 30% hydrochloric acid is fed into both circuits in order to strengthen the weak acid to 20-26% (Winnacker Kuchler: Chemische Technologie, volume 2, 4th edition, 1982, pages 443 ff).
A disadvantage of the diaphragm process is the fact, the anolyte and catholyte mix each other through the diaphragm, which cannot be controlled from outside, because the diaphragm is permeable to the electrolyte. In consequence the chlorine which has been dissolved in the anolyte acid is partly stripped off after it has penetrated onto the catholyte side, and thus leads to contamination of the hydrogen. Another part is reduced cathodically and thus leads to a reduction in the product/energy yield. A partial mixing of the gases, chlorine and hydrogen, which have been produced in the electrolysis process, can also occur through the diaphragm.
These disadvantages can be obviated by the use of an ion exchange membrane instead of the diaphragm. Thus, it has been suggested, in DE-A 2 844 499, that an ion exchange membrane, which has been coated with electrocatalysts on both sides be used, and that the current be supplied or withdrawn by feeder and collector electrodes. This process, which is described as the "solid polymer electrolyte" (SPE) system, has the advantage that only one electrolyte circuit is required, since the protons which are discharged on the cathode side migrate from the anode side through the membrane; thus, theoretically, no depletion with respect to ions takes place in the catholyte. However, in practice, hydration water goes over to the cathode side, and this must be removed.
One disadvantage of this process is, however, the fact that the current transfer from the feeder/collector electrodes onto the "working electrodes" is inside the electrolyte and thus defined current transfers can be adjusted only with difficulty and cannot be controlled from outside.
Another disadvantage in DE-A 2 844 499 is that the metal meshes which are proposed as feeder/collector electrodes are stable in industrial application only for a very short time on the anode side and only under cathodic protection on the cathode side. Corresponding mashes made of graphite which would be corrosion resistent are very expensive and are not very mechanically stable, because of the size of industrial electrolysors.
Reduced oxides from the group of nobel metals are used as electrocatalysts and are mixed with graphite to a greater or lesser extent. These systems are, however, far less stable than graphite under operating conditions. If, however, graphite has to be used for the feeder/collector electrodes simply for reasons of durability and also for the electrocatalytic layers on the membranes, the solid graphite electrodes of the original kind present no longer any significant disadvantages. Moreover, the problems which constantly occur in the adhesion of the electrocatalyst to the membrane are avoided.
If, however, the electrodes become separated from the membrane, a conductive electrolyte is also required on the cathode side. The maximum conductivity of aqueous hydrochloric acid is, as is well-known, at a level of concentration of between 17 and 22% by weight HCl. Since the concentration of HCl solution in the cathode region decreases as a result of the transfer of hydration water through the membrane the HCl solution must be replaced.